Thin film electroluminescent (TFEL) display panels offer several advantages over other display technologies such as cathode ray tubes (CRT) and liquid crystal displays (LCD). Compared with cathode ray tubes, TFEL display panels require less power, provide a larger viewing angle, and are much thinner. Compared with liquid crystal displays, TFEL display panels have a larger viewing angle, do not require auxiliary lighting and can have a larger display area.
Prior art TFEL display panels such as the panel shown in FIGS. 1 and 2 are generally built on a glass substrate 1. A plurality of parallel transparent electrodes 2 are deposited on the glass. A first layer of dielectric 4, a phosphor layer 6 and a second dielectric layer 8 are deposited over the transparent electrodes 2. A plurality of metal electrodes 5 are applied on top of the second dielectric layer perpendicular to the transparent electrodes 2. The transparent electrodes 2 are typically indium-tin oxide (ITO) and the metal electrodes 5 are typically aluminum. The dielectric layers 4 and 8 protect the phosphor layer 6 from excessive dc currents. When an electrical potential, such as about 200 volts, is applied between the transparent electrodes 2 and the metal electrodes 5, electrons tunnel from one of the interfaces between the dielectric layers 4 and 8 and the phosphor layer 6 into the phosphor layer where they are rapidly accelerated. The phosphor layer 6 typically comprises ZnS doped with Mn. Electrons entering the phosphor layer 6 excite the Mn causing the Mn to emit photons. The photons pass through the first dielectric layer 4, the transparent electrodes 2, and the glass panel 1 to form a visible image.
Speed and brightness uniformity of the electroluminescent display depend critically on the ITO line resistance, particularly for large area displays. Even with integrated ITO lines, a zebra pattern brightness contrast occurs as a result of high resistance. Therefore, to achieve higher conductivity of the transparent electrodes in the electroluminescent display panel, an improved structure was developed in which the low conductivity ITO electrodes were augmented by buses of thick, narrow, high conductivity metals. This improved electroluminescent display is discussed by O. J. Gregory et al. in their article Fabrication of High Conductivity, Transparent Electrodes with Trenched Metal Bus Lines, Journal Electro-Chemical Society, Vol. 138, No. 7, July, 1991. In the most common augmented ITO panel an assist structure is on top of each ITO electrode and adjacent the overlaying dielectric layer. In this structure there is a electromagnetic field radiating from the metal into the dielectric. As a result of these fields we have observed that the large electroluminescent panels were failing at the edges. In their article Gregory et al. disclose an augmented ITO electrode in which the metal conductor is recessed into the glass substrate. This structure does not have the deleterious electrical field problem of the prior augmented electroluminescent display panels. However, to make the panel disclosed by Gregory et al. one must etch parallel paths into the glass substrate corresponding to the desired positions of the metal assists. Then the metal must be deposited into the grooves which have been etched into the glass. Etching the glass adds an additional step to the production. Additionally, making such grooves weakens the glass which can cause the glass to crack or fail along those cut paths. Therefore, it is preferable to place a metal assist on top of the glass or a transparent electrode deposited thereon.
We have observed the electric field between a metal assist placed on a transparent electrode and the adjacent dielectric layer. We saw that the strength of that electric field is related to the sharpness of the corner on the metal assist structure. We, therefore, conceived that the problem of high electric fields at the edge of the metal assist structure could be minimized by rounding the corners of each metal assist structure. However, a metal assist structure with rounded corners would be difficult and expensive to fabricate. Accordingly, this is not a practical solution.
Thus, there is a an improved electroluminescent display panel which utilizes a metal assist structure, but does not suffer from the problems created by high electric fields generated at the edges of the metal assist structure. Moreover, such an improved electroluminescent display must be easy to manufacture, preferably adding no significant cost to the current process for making electroluminescent display panels. Moreover, such an improved panel should be able to be made using conventional manufacturing techniques.